Electrolyte composition for electropolishing niobium and tantalum and method for using same

ABSTRACT

The electrolyte composition and electropolishing method for electropolishing surfaces of metals and their alloys, in particular surfaces of niobium, niobium containing alloys, tantalum, tantalum containing alloys is disclosed. The electrolyte composition comprises methanesulfonic acid and hydrofluoric acid. Said electrolyte composition make possible to electropolish niobium, niobium containing alloys, tantalum and tantalum containing alloys to the best quality ever by eliminating problems of hydrogen absorption and sulfur contamination o the workpieces.

BECKGROUND OF THE INVENTION

The electropolishing process is almost 100 years old electrochemical process applied to metals, metal alloys and intermetallic compounds in purpose to smooth the surface (by minimizing macro and micro roughness), make the workpiece surface shiny and reflective, improve bio and hemocompatibility (metallic body implants), to remove the stressed and deformed layer (Beilby layer), make surface pure and homogenous etc.

Electropolishing processes mainly use direct current (DC), the exception are platinum metal group, which are elecvtropolished by using alternating current (AC).

The hundreds of different electrolytes composition are used to electropolished various metals, alloys and intermetallic compounds. Mainly all of those electrolytes consist of mixture of very strong concentrated inorganic acids, or inorganic acids with organic acids, alcohols, salts and another additives. Also some electropolishing processes are based on basic electrolytes as for example sodium hydroxide electrolyte for electropolishing tungsten and tungsten alloys.

In recent two decades the electropolishing process seems to be rediscover. This is due to growing demands for ultra pure, corrosion resistant, bio and hemocompatible, stressed deformed layer and grain boundaries free super smooth metal surfaces.

One of the field that surfaces of metal are so critical to their performance are superconductive niobium cavities. To perform on the highest level possible the niobium cavities are electropolished.

The main reason to electropolish niobium cavities is to remove stressed deformed layer around ≈150 microns thick created during forming operations. The uncovered by electropolishing surfaces have to be as smooth as possible, free from any contaminations specially sulfur precipitates and hydrogen free as possible.

The present state of the art method employed for electropolishing niobium cavities in all laboratories around the world uses the very old electrolyte formula (first mention by W. Tegart in 1956 as electrolyte for electropolishing titanium) of mixture of concentrated hydrofluoric acid and sulfuric acid in 1:9 proportion.

The above formula was adopted by Siemens company in the 70's for electrropolishing niobium cavities and is commonly refereed as Siemens process.

The above process poses lot of disadvantages, which demand lot of post electropolishing operations:

1. sulfur contamination (nature of electrolyte—decomposed sulfuric acid leaves sulfur precipitates 2. hydrogen uptake (aggressiveness of electrolyte toward niobium during absence of applied current—filling and empting of electrolyte) 3. slow rate of dissolution around 0.4 microns per minute 4. short live of electrolyte due to loses of hydrogen fluoride by evaporation (high concentration of sulfuric acid ≈98%)

The above disadvantages specially sulfur contamination and hydrogen absorption demand very costly and time consuming post electropolishing operations as special chemical soaking and washing in ethanol and vacuum baking respectively.

Some work have been done to develop new electrolytes to overcome such problems: as sulfur contamination, hydrogen absorption, dangerousness of electrolyte (by trying to eliminate the hydrofluoric acid), speeding the dissolution rate and improve surface smoothness. Unfortunately any of them were successful enough to substitute currently used one.

Some of those new proposed electrolytes were mixtures of more than two ingredients, which made them more complex and by this more difficult to use in reproducible way.

Other ones by substituting hydrofluoric acid by fluoride salts hypothetically making them safer have compromised their effectiveness.

BRIEF SUMMARY OF THE INVENTION

The present invention relates to electrolyte composition and method for using same for electrochemical polishing of surfaces of metals and metal alloys, in particular of metals and metal alloys that are selected from group comprising of niobium, niobium alloys, tantalum and tantalums alloys.

Said electrolyte contains methanesulfonic acid (MSA) and hydrofluoric acid (HF).

DESCRIPTION OF THE INVENTION

The present invention relates to bath composition and method for electropolishing metal surfaces consist of niobium, niobium alloys, tantalum and tantalum alloys. The electropolishing bath comprises solely of methanesulfonic acid and hydrofluoric acid, which formula is not known to prior art.

The use of methanesulfonic acid giving lot of advantages to electropolishing niobium, tantalum and their alloys. The main advantage is decreased aggressiveness of the electrolyte toward niobium and tantalum when compared to present state of the art electrolyte consisting of 1:9 hydrofluoric acid and sulfuric acid.

This means that that electrolyte according to present invention does not react with niobium or tantalum surfaces chemically during the time when applied current is switched off (start and finish of electropolishing process). By this hydrogen creation, which could adsorb on the surface of workpiece and dissociate to atomic hydrogen which could diffuse into the metal by absorption mechanism (one of the main problems in electropolishing niobium cavities) is prevented.

Another advantage of using electrolyte of present invention is elimination of possibility of sulfur deposition on the surface of workpiece (which is very detrimental in the case of niobium cavities and have to be removed by soaking and washing in ethanol).

The elimination of sulfur deposition during electropolishing operation and prevention of hydrogen absorption are of paramount importance in preparation of niobium cavities. They are the main culprits of problems with field emission and possibly quench.

Also the highest rate of dissolution without sacrificing surface smoothness is achieved by using electrolyte according to present invention. The rate of dissolution can be around 15 times higher when compared to currently used state of the art formula 1:9 hydrofluoric acid and sulfuric acid and can reach 6 μm/min.

The above advantages are coming from the properties of methanesulfonic acid. It is well established that methanesulfonic acid has very high electrolytic stability, excellent conductivity, high dissociation rate, very low level of sulfates, low vapor pressure, any aggressiveness toward niobium (without applied current) and is very temperature stable.

Another advantage of using methanesulfonic acid are safer working condition for people conducting the process (mixing methanesulfonic acid with hydrofluoric acid don't trigger exothermic reaction, no splashing, bubbling or excessive fuming). Less fuming of the electrolyte of present invention giving it prolong life without need to frequent replenishing with hydrofluoric acid.

Disposal of spent electrolyte has to be taken also under consideration. The disposal of the spent electrolyte according to the present invention is easier and safer for the environment, because methanesulfonic acid is non oxidant, not toxic and readily biodegradable.

The usefulness of the present invention is shown on following examples. The examples used represent only possible embodiments of the electropolishing process described here and should not in any way imply any restriction to the condition used here.

EXAMPLES Example 1 Electropolishing of Niobium

Prior to electropolishing coupons of pure niobium was ultrasonically cleaned in methanol, washed in water and dried. The following electropolishing parameters were used.

material: squares coupons 10×10×5 mm

electrolyte: 70 vol % methanesulfonic acid+30 vol % hydrofluoric acid

bath temperature: 25° C.

applied voltage: 10V

time: 1 hr

After electropolishing the coupons were rinsed in water and dried.

Results: very shiny and reflective surface were achieved of average roughness Ra 0.06 μm, dissolution rate 6 microns/min.

Example 2 Electropolishing of Tantalum

Prior to electropolishing pure tantalum wire were rinse in ethanol and dried. The following electropolishing parameters were used:

material: 0.5 mm diameter pure tantalum wire

electrolyte: 80 vol % methanesulfonic acid+20 vol % hydrofluoric acid

bath temperature: 40° C.

applied voltage: 15V

polishing time: 30 minutes

After electropolishing wire was rinsed in water and dried.

Results: shiny, smooth surface was achieved. 

1. The electrolyte for electropolishing of surfaces of metals and metals alloys that are selected from group comprising niobium, niobium alloys, tantalum and tantalum alloys, wherein the electrolyte composition comprises methanesulfonic acid 70 wt % and hydrofluoric acid 48 wt %.
 2. The electrolyte as claimed in claim 1, wherein it contains methanesulfonic acid in concentration of from 50 to 95 vol %, preferably 70 to 80 vol %.
 3. The electrolyte as claimed in claim 1, wherein it contains hydrofluoric acid in concentration of from 5 to 50 vol %, preferably 20 to 30 vol %.
 4. The electrolyte as claimed in claim 1, wherein the electrolyte does not contain any other ingredient.
 5. A method for electropolishing workpieces of niobium, niobium alloys, tantalum and tantalum alloys, wherein an electrolyte as claimed in claim 1 is used.
 6. The method as claimed in claim 5, wherein electropolishing is carried out at temperature between 10° C. and 50° C., preferably 25° C.
 7. The method as claimed as in claim 5, wherein electropolishing is carried out using direct current (DC) of constant voltage between 5 V to 15 V, preferably 10 V. 